1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine that includes a compression ratio change device that changes a mechanical compression ratio, wherein the mechanical compression ratio is a ratio of a maximum value of a volume of a combustion chamber to a minimum value of the volume of the combustion chamber, and the volume of the combustion chamber is changed according to the reciprocating movement of a piston.
2. Description of the Related Art
A control apparatus for an internal combustion engine that includes a compression ratio change device is available. The compression ratio change device changes a mechanical compression ratio of the internal combustion engine. Generally, when the temperature and pressure of mixed gas (end gas) located near a wall surface constituting a combustion chamber (for example, a bore wall surface) are extremely high, the end gas is strongly burned after the mixed gas is ignited. Thus, knocking occurs. When knocking occurs, the heat efficiency of the internal combustion engine is greatly decreased, and the wall surface constituting the combustion chamber (for example, a cylinder head) may be molten and damaged.
As the load of the internal combustion engine decreases, an amount of fuel in the mixed gas formed in the combustion chamber is decreased, and therefore, heat energy generated by combustion of the mixed gas is decreased. As a result, the wall surface constituting the combustion chamber is heated by the heat energy generated by combustion of the mixed gas, to a smaller extent, and therefore, the end gas is heated by the wall surface to a smaller extent before the end gas is burned. Further, as the load of the internal combustion engine decreases, the amount of mixed gas formed in the combustion chamber is decreased, and therefore, the temperature of the mixed gas is increased to a smaller extent when the mixed gas is compressed by the piston. Accordingly, as the load of the internal combustion engine decreases, knocking is less likely to occur.
Thus, Japanese Patent Application Publication No. 2006-177176 (JP-A-2006-177176) describes an example of the control apparatus that controls the compression ratio change device to increase the actual mechanical compression ratio as the load of the internal combustion engine decreases. Accordingly, when the load of the internal combustion engine is relatively low, the mechanical compression ratio is controlled to be relatively high. This increases the heat efficiency of the internal combustion engine. As a result, it is possible to operate the internal combustion engine with good fuel efficiency, while avoiding occurrence of knocking.
Further, when knocking occurs, the control apparatus controls the compression ratio change device so that the actual mechanical compression ratio is lower than the mechanical compression ratio set based on the load. This decreases the pressure of the end gas, thereby suppressing occurrence of knocking.
However, the above-described conventional control apparatus continues to control the actual mechanical compression ratio to the mechanical compression ratio that is lower than the mechanical compression ratio set based on the load, after knocking occurs. Therefore, an increase in the fuel efficiency may be inhibited.